Permanent magnets are used to an increasing extent in large electrical machines, for instance in motors, generators and other such machines. This is due to the increased efficiency and robustness compared with electrical excitation. In particular, rare-earth magnets, primarily based on neodymium iron boron (abbreviated as NdFeB), have turned out to provide a very high energy product and are therefore very useful in compact machinery.
However, some problems in relation to the practical application remain unsolved. The best magnet materials corrode very easily, need a high degree of protection and are also rather brittle and cannot safely be fixed by bolting alone. Furthermore, the known manufacturing methods comprise a long series of steps, some of which are expensive and involve wastage of costly materials.
The traditional method of mounting permanent magnets on, for example, a large generator rotor comprises extensive surface protection on the individual magnets and gluing the magnets to the rotor rim. Furthermore, it is necessary to wrap the completed rotor with glued-on magnets with a fibreglass bandage.
This method contains several difficulties. The surface protection is extensive and due to new technologies it is not proven over a lifetime of, for example, 20 years. The magnets cannot be magnetised in situ. This means that all work is done with magnetised parts which require special tools and stringent control of the work to avoid hazardous situations. Once mounted on the motor and covered by a fibreglass bandage the magnets cannot be removed for re-magnetising in the case of an irreversible demagnetisation event.